Various types of instability may be encountered during operation of a controlled system. Typically, a controller can generate reference command signals for transmission to various actuators to define positions of the actuators during the operation of the controlled system. Feedback received from the controlled system can include actual positions of the actuators in operation. These actual positions can be compared to the command signal to detect any discrepancies. The detected discrepancies may indicate that some instability exists and are used to declare errors.
FIG. 1 illustrates how, in some conventional systems, errors are declared based on predetermined thresholds and a period of time the instability lasts. As shown in FIG. 1, in operation 105, a controller provides a reference command signal to an actuator, and in operation 110, the controller monitors the feedback signal including the actual position of the actuator. In operation 115, the command signal and the feedback signal are compared to each other to determine whether any discrepancies are present and, if such discrepancies are present, it is determined whether the feedback signal exceeds thresholds in operation 120. When the feedback signal is within the thresholds, monitoring of the feedback signal continues. When the feedback signal exceeds any of the thresholds, a delay timer is activated in operation 125. The delay timer is incremented until the feedback signal exceeds thresholds.
The feedback signal is monitored for returning within thresholds in operation 130. In operation 135, the feedback signal is compared to the thresholds. When the feedback signal returns within the thresholds, the delay timer is reset in operation 140, and no error is declared. If the feedback signal does not return within thresholds in operation 135, the delay timer continues incrementing, until the predetermined value is reached. Thereafter, the delay timer expires in operation 145, and a steady state error is declared in operation 150.
FIG. 2 illustrates steady state error detection 200 by showing a signal in relation to time. A command reference signal defines a reference position 230 of a positioning device, such as an actuator, whereas a feedback signal shows an actual position 205 of an actuator. An upper threshold 215 and a lower threshold 210 determine acceptable deviations of the actual position 205 from the reference position 230. When the actual position 205 exceeds any thresholds, a delay timer starts incrementing 220 and increments until either the delay timer expires, or the actual position 205 returns within thresholds. When the delay timer expires, a steady state error is declared. If the actual position 205 is back within the thresholds before the delay timer expired, no error is declared.
Thus, conventional methods may allow detecting steady state errors. However, steady state errors are only one type of error encountered by actuators. Other types of errors include oscillatory type errors, which are characterized by fluctuations of the actual position of a positioning device in relation to the reference position or a steady state error. The oscillatory type errors may not be detected by the conventional methods described above, which simply reset the timer when the actual position 205 returns within the thresholds.
However, oscillatory errors, when severe enough, can lead to deviations from the desired system state, damage to system components, and potential system instability as well as poor performance.